restoreView() refuses to reopen the popup ONTO a non-restorable view, but
the stack behind it was restored verbatim, so a screen the popup will not
render -- export-privkey, show-phrase -- could sit in it. Back then landed
on a view whose content is deliberately never re-rendered, and show-phrase
has no Back control to leave by. No secret is exposed: those screens are
empty precisely because nothing is restored into them. This is navigation,
not disclosure.
loadState() now truncates the stored stack at the first entry outside
RESTORABLE_VIEWS rather than splicing that entry out, so the result stays a
prefix of what was stored and every surviving entry keeps exactly the Back
target it had; splicing would silently re-point the entry above the hole at
a different screen. A stack truncated to nothing under a restorable
non-root view gets main beneath it, so Back always has somewhere to go.
Stacks with no unrenderable entry are restored unchanged.
The filter is on load, not on save: the live in-session stack is
legitimate, since the screen really is rendered while the popup is open,
and only a load-side filter also repairs the stacks already in storage,
including ones written before a view left the set.
